I have heard every speaker on this list apart from the DSP3200 which was added later by admin. I have owned and still own quite a few of these speakers and dabbled extensively with center matching since I'm quite critical of the center speaker, it's often the weakest link of the system and when it isn't it's simply an outstanding Trifield system.
As you see, the VC's are rated higher than their HC counterparts, they match and blend better.
With the smaller DSP5k/DSP5.2k's it's not just that they are horisontal but actually are different speakers with completely different cabinet volumes from their vertical siblings. That means radically different speakeroutput and lowend capability. Try putting a DSP5k/5.2kHC instead of your DSP5k/5.2k front mains L/R position and you will probably hear what I mean.
For example with Veritcal 7200 main speakers a Vertical 6000 might work better than a Horizontal 7200HC center. I'm not saying that IS true. I'm just saying it could be true. Because both the 7200HC is different as well as a 6000C. Cosmetics is a whole other story.
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Also even a vertical 5200 with for example 8K mains might do better than any Horizontal speaker. Or a Vertical 7000 might do better than a 7200HC with 7200 mains. Obviously I have not tested every combinination possible nor do I think any one has. I'm just saying it's possible the rule is not so simple.
I think it's almost as simple as this list if adequate placement can be accomodated for each alternative.
Matched height alone is a factor too. A raised 7200HC might be just as good as a 7200 Vertical.
There is definately an improvement to raise the center speaker to tweeter/mid of the mains. But HC version will always be behind and never surpass it's VC counterpart.